The invention resides in the field of wireless communication devices and systems where a wide variety of signals need to be handled. In particular it relates to transconductance cells for amplifying/filtering wireless signals, which cells have a high linearity over a wide range of signal levels and yet require a low operational voltage.
In typical applications, a wireless receiver must operate over a wide range of signal levels as well as large interfering signals. The interfering signals emanate from users in adjacent channels, as well as from transmission sources which may be relatively far removed in frequency but have a large transmission power. When two interfering signals at frequencies f1 and f2 (where f1 and f2 are close to the desired signal frequency) are present, they will producexe2x80x94due to amplifier nonlinearityxe2x80x94intermodulation products at frequencies 2f2xe2x88x92f1 and 2f1xe2x88x92f2. These can fall at frequencies close to the desired signal frequency. The resulting interference causes measurable degradation of the bit error rate (BER) in digital communication systems.
The ability of circuits to handle large signals can be characterized by the third-order intercept point (IP3) which is a measure of circuit linearity. In most receivers, signal filtering circuits and variable gain control circuits are essential parts of signal processing to decode the transmitted information. These circuits often use transconductance amplifiers/filters as building blocks as they satisfy the requirements well. The transconductance amplifiers/filters are in the configuration of differential amplifiers and are also called simply differential amplifiers.
Following articles describe in detail requirements of wireless communication receivers and suggest some design principles of transconductance amplifier/filters.
[1] Fenk J. and Sehrig P.: xe2x80x9cLow-noise, low-voltage, low-power IF gain controlled amplifiers for wireless communications,xe2x80x9d in Analog Circuit Design, Huijsing J. H. et al. (eds), 1996 Kluwer Academic Publishers, pp. 27-44.
[2] Crols J., Steyart M.: xe2x80x9cLow-IF Topologies for High-Performance Analog Front Ends of Fully Integrated Receivers,xe2x80x9d xe2x80x9cIEEE Transactions on circuits and systems-II: Analog and digital signal processing,xe2x80x9d Vol. 45, No. 3, March 1998, pp. 269-282.
There is a further requirement however of these signal filtering and variable gain control circuits and that is that they must be powered at a very low voltage.
An article below describes cascaded current mirror circuits which permit low voltage operation of transconductance amplifiers.
[3] Crawley P. J., Roberts G. W.: xe2x80x9cDesigning Operational Transconductance Amplifiers For Low Voltage Operationxe2x80x9d xe2x80x9cIEEE International Symposium on Circuits and systemsxe2x80x9d Chicago, Ill., May 1993, pp. 1455-1458.
Following U.S. Patents describe a variety of transconductance amplifiers: U.S. Pat. No. 5,444,414 Aug. 22, 1995 Delano, U.S. Pat. No. 5,451,901 Sep. 19, 1995 Welland and U.S. Pat. No. 5,844,442 Dec. 1, 1998 Brehmer.
In spite of the prior art mentioned above, there are pressing needs to have a transconductance amplifier/filter that can operate at a low supply voltage and yet exhibit a high linearity. Some embodiments of the invention include a common mode feedback circuit used to bias the transconductance amplifier and/or a variable gain circuit to permit operation at a different gain settings.
In yet another embodiment, a complex filter cell includes four Gm transconductance cells to realize a complex pole filter. More poles can be realized using cascaded complex Gm cells. A wireless communications receiver including such filters is also described.
The specification will describe the invention and its advantages in full in connection with circuits which use bipolar transistors. It should, however, be noted that any active devices including e.g., MOS etc., can be used to implement the present invention to realize the advantages.
According to one aspect, there is provided a low voltage transconductance cell, having a high linearity. The cell comprises a transconductance core for differential inputs and differential outputs, current sources for providing operational currents for the transconductance core, current mirror circuits connected to the transconductance core for generating a pair of mirrored feedback currents to be fed back to the current sources, a bias circuit for setting an operational point of the transconductance core, and a common mode feedback circuit connected to the outputs of the transconductance core for generating a bias signal to be applied to the bias circuit. The common mode feedback circuit includes an averaging circuit for averaging the differential outputs and a comparing circuit for comparing an average of the differential outputs with a reference value to generate the bias signal.
According to a further aspect, there is provided a low voltage transconductance cell having a high linearity, which includes: a differential amplifier stage including a non-inverting section and an inverting section the non-inverting section having a non-inverting input and a non-inverting output and the inverting section having an inverting input and an inverting output; first and second current sources generating a current flow through the non-inverting section and the inverting section respectively; first current mirror and second current mirror circuits connected to the non-inverting and inverting sections respectively, the first current mirror circuit sensing the current in the non-inverting section and generating a first mirror current to feed back to the second current source, and the second current mirror circuit sensing the current in the inverting section and generating a second mirror current to feed back to the first current source.